The field of art to which the claimed invention pertains is alumina particles. More specifically, the claimed invention relates to a novel method of preparing alumina particles by the oil-drop method wherein no oil aging step is required, the total aging time of the alumina particles is substantially less than the total aging time required by conventional practice, and the alumina particles of this invention are substantially stronger than alumina particles produced by conventional practice.
The physical and structural properties of a catalyst influence significantly its activity and durability. To a substantial degree, the physical and structural properties of a catalyst are determined by the physical and structural properties of the catalyst support or base material. The pore structure, including the pore size distribution and pore volume, determines to a large degree the extent and accessibility of surface area available for contact of the catalytic material and the reactants. Catalytic activity is often a function of the rate of diffusion of reactants and products in and out of the interstices of a catalyst. Thus, increased pore size may facilitate the diffusion of reactants and reaction products and consequently result in increased activity. However, pore size alone does not influence catalytic activity. Catalytic activity is a function also of surface area available as a reaction site. Thus, it is desirable to obtain a catalyst with sufficient surface area and also satisfactory pore sizes.
Other physical and structural characteristics which are important for many catalysts are low density and high crush strength and attrition resistance. Low density catalysts generally are desired since low density catalysts are able to respond more rapidly to temperature changes and consequently require less time and energy to reach reaction temperature. High crush strength and attrition resistance is particularly desired of catalysts which are used in large volumes or in moving bed operations.
Alumina is a particularly desirable catalyst support material since it is of high porosity and surface area, and is structurally stable over a wide temperature range.
Catalyst particles have been manufactured in a multiple of physical shapes, the most common macrosized (about 1/32" to about 1/8") particles probably being cylinders and spheres. Spheroidal shaped catalyst particles, and especially spheroidal alumina particles, have many advantages over particles of other shapes particularly when employed as a catalyst or as a catalyst support or carrier material in a fixed bed type of operation. When so employed, such particles permit a more uniform packing whereby variations in pressure drop across the bed are minimized, and the tendency of a reactant stream to channel through the bed out of effective contact with the catalyst is substantially obviated.
In many applications, the performance of spheroidal alumina particles, either as a catalyst or as a catalyst support, is judged not only on their activity, activity stability, selectivity and selectivity stability with respect to a particular conversion process, but also on their physical stability or durability. Physical stability is of particular importance in applications where catalyst particles are subjected to vibration and general movement in a reactor or converter. Although the average particle strength may be quite acceptable, it is the disintegration of the relatively weak particles which leads to catalyst loss and the formation of fines which accumulate to plug retaining screen and effect an undue pressure drop across a catalyst bed. In addition, the disintegration of weaker particles of a tightly packed bed promotes excessive movement of the remaining particles in contact with each other resulting in further loss of catalyst through abrasion.
It appears that there are generally five methods of preparing spheroidal particles of a size suitable for commercial catalytic purposes. The oldest and least satisfactory method is the pilling operation, whereby irregularly shaped particles are operated upon mechanically, as by agitation in contact with like particles or other objects, to produce generally spheroidal particles. This method tends to produce particles of relatively non-uniform size and shape. In addition, large amounts of attrition products are produced. Finally, the operation requires significant capital investment in equipment and is energy intensive.
Another method of mechanically preparing generally spheroidal particles is by the Marumerizing technique, whereby particles are discharged onto a rotating inclined pan or disc to effect agglomerating spheroidizing of the particles. This technique suffers from many of the same disadvantages as the pilling operation.
A third method of preparing spheroidal particles is the spray drying method, whereby a solution or a slurry is sprayed through an orifice under conditions to produce numerous very small particles. This method is not suitable for the production of macrosized particles in the range of from about 1/32 inch to about 1/2 inch.
Two other methods of producing spheroidal alumina catalysts have been developed. Both methods involve the dropping of an alumina containing material into a water-immiscible liquid to form spheroidal particles as a result of surface tension interaction. However, the methods are quite distinct both in the alumina containing material used and in the results obtained.
One of these two methods comprises preparing crystalline alumina slurry, passing drops of the slurry through a water-immiscible liquid to form spheroidal particles, and simultaneously or thereafter passing the particles through a setting solution to rigidify the exterior surface of the spheres. (U.S. Pat. No. 3,558,508; U.S. Pat. No. 3,943,070; U.S. Pat. No. 4,179,408). In one embodiment of this method, an alumina slurry, a significant component of which is solid crystalline alumina, is prepared by admixing alumina of extremely small particle size with a small amount of a non-oxidizing acid and water. In another embodiment of the same method, the crystalline alumina slurry is prepared by commingling a precipitated alumina and an acidic aqueous medium. The general method can be characterized as follows. First, the crystalline structure of the ultimate catalyst is selected. Second, a slurry is prepared which contains substantially all or at least a significant portion of its alumina content in the same crystalline form as the desired ultimate catalyst. Third, a method of processing the slurry to form spheres is selected to minimize the disruption or change in crystalline structure of the alumina in the slurry so that the crystallized form of the alumina in the slurry is retained in the ultimate catalyst. In all cases the slurry comprises two phases, a liquid phase and a solid phase, a substantial portion of the latter comprising crystalline alumina. It appears to be necessary to minimize or at least maintain at low levels the amorphous alumina content of the slurry to promote the formation of firm spheres. Relatively small amounts of acid are used so that the solids content of the slurry is maintained and the crystalline nature of the alumina is not destroyed. The catalyst particles resulting from use of a slurry are believed to comprise a substantial amount of mechanically retained crystalline alumina particles due to the small amounts of acid used in the preparation of the slurry.
The other general method of producing spheroidal alumina particles by dropping an alumina-containing material in a water-immiscible liquid comprises the use of an amorphous alumina hydrosol. (U.S. Pat. No. 2,620,314; U.S. Pat. No. 3,919,117; U.S. Pat. No. 3,887,493). Instead of a slurry containing solid crystalline alumina, this method comprises use of a liquid or colloidal suspension of amorphous alumina without solids content. It is disclosed that the alumina hydrosol can be prepared by the hydrolysis of an acid salt or aluminum, as, by the digestion of aluminum metal under heat by an aqueous solution of aluminum chloride.
It is obvious that the method of producing spheroidal alumina particles from an alumina hydrosol is superior in many ways to the previously mentioned method which comprises the use of a crystalline alumina slurry. However, a persistent problem in the practice of the method comprising the use of a hydrosol has been the inability of the formed spheroidal particles to maintain their structural integrity if immediately removed from the water-immiscible liquid. Consequently, it has been the practice to age the particles in the water-immiscible liquid prior to their removal. The method of this invention eliminates the necessity of such an aging period.
Heretofore, the method of preparing spheroidal alumina particles from an alumina hydrosol has comprised commingling an alumina sol and a gelling agent at below gelation temperature and dispersing the mixture as droplets in a water-immiscible suspending medium, usually a gas oil, maintained at an elevated temperature whereby the hydrosol droplets are formed into firm, spherical, hydrogel particles. However, the practice has not been to immediately remove the particles from the water-immiscible liquid. Rather, it has been the practice to retain and age the hydrogel spheres in the oil suspending media for an extended period, and thereafter in an aqueous akaline media for a further extended period. The oil aging process has heretofore been considered as essential to obviate excessive cracking and sphere disintegration during the subsequent aqueous phase treatments.
It has been disclosed that the physical stability of spherical alumina particles prepared by the oil-dropping method can be enhanced by a two-step aging process using varying concentrations of ammonium hydroxide. (U.S. Pat. No. 4,108,971). Ammonia-yielding compounds also are disclosed to be useful in preparing extrudates having certain desirable pore characteristics. (U.S. Pat. No. 4,048,295). It also has been disclosed that in the preparation of amorphous alumina spheroidal particles from alumina hydrogels, cracking and sphere disintegration is caused by osmotic swelling of the hydrogel spheres resulting from the salt concentration gradient between the aqueous phase of the hydrogel spheres and the external aqueous phase. (U.S. Pat. No. 3,887,492). The solution to the problem has been disclosed to be to immerse the hydrogel particles in an aqueous solution having a pH of at least about 5.5 and a salt concentration substantially equivalent to the salt concentration of the internal aqueous phase of said hydrogel particles; maintain the particles in contact with the solution while reducing the salt concentration gradually at conditions to minimize the concentration gradient between the solution and the internal aqueous phase of the particles until the solution is substantially salt-free; separate, and dry and calcine the resulting hydrogel particles to form amorphous alumina spheres.